It is known to provide vessels having a moonpool and a hoisting device adjacent the moonpool with a mobile working deck, i.e. a working deck that in working position covers the moonpool and allows workmen to approach the firing line passing through the working deck and the moonpool, and in another position is removed from the moonpool to enable the transport of equipment through the moonpool.
Typically, a mobile working deck is a single part, removable working deck. These working decks can be moved, for example using a hoisting device, between a storage position and a working position. Typically, the storage position is located some distance away from the moonpool, such that transport of the working deck takes a significant amount of time. When the working deck is stored close to the moonpool it often obstructs access to the moonpool when in the storage position.
It is furthermore known to provide a pivotable working deck, which working deck is pivoted between a working position in which it covers the moonpool and an upright storage position in which it is located adjacent the moonpool. Although the pivoted position allows for moving equipment into the moonpool, the pivotable deck also forms an obstruction and thus allows for limited transport only.
In WO2009/102197, which is incorporated herein by reference, a vessel comprising a moonpool is disclosed. The monohull drilling vessel is furthermore provided with firing line hoist systems mounted on the hull. The firing line hoist systems comprises:                a tower, which tower has a top side and a base, a first side and a second side, and wherein the base of the tower is connected to the hull of the drilling vessel at opposite sides of a moonpool, such that the first side of the tower and the second side of the tower bridge the moonpool in a lateral direction thereof, and        a hoisting device, which hoisting device comprises a load attachment device displaceable supported by the tower, one or more cables and one or more associated winches to manipulate the position of the load attachment device relative to the tower in a vertical direction along a firing line, which firing line extends on the outside of and adjacent to the first side of the tower and through the moonpool, and which hoisting device is adapted for raising and lowering a BOP (Blow Out Preventer) to the seabed;        
The vessel is furthermore provided with:                a BOP transport system, configured for moving a BOP between a storage position and a launch position, in which launch position the BOP is located in the firing line adjacent the first side of the tower, and in which launch position the BOP can be manipulated by the hoisting device, wherein the BOP transport system comprises a BOP track with two parallel guide rails and a BOP transport cart which is movably supported by those guide rails; and        a mobile working deck that is provided at the first side of the tower, and which working deck in an active position extends in a substantially horizontal direction and covers a portion of the moonpool, while the firing line extends through the working deck, and in a non-active position provides room for a BOP to be moved by the BOP transport system into a launch position above the moonpool.        
By arranging the mobile working deck in a mobile manner the deck can be in a relative low position with respect to the waterline when work has to be carried out on the working deck, e.g. assembly of a riser string or a drill string. The mobile working deck is removed when a BOP is to be positioned in the firing line. After the BOP has been positioned in the firing line, and has been lowered, the mobile working deck is returned into its active position.
In case the mobile working deck is a sliding working deck, the mobile working deck is supported by the BOP track such that it can be slid along the BOP track in a direction perpendicular to a longitudinal axis of the moonpool between a first position in which the working deck covers a portion of the moonpool and the firing line extends through the working deck, and a second position adjacent the moonpool, in which the working deck provides room for a BOP to be moved by the BOP transport system into a launch position above the moonpool. After the BOP is brought into the firing line it is lowered onto a cart positioned above the moonpool. The mobile deck is replaced and the BOP is lifted and suspended from a suspension device of the working deck. Subsequently a riser section is connected to the top end of the BOP, and the BOP and riser section are lowered by means of hoisting device into the sea. This is an elaborate process.
The configuration with a deck that is slideable supported by the BOP track, allows for one BOP to be stored at one side of the moonpool and the working deck to be parked on the opposite side of the moonpool.
In an alternative embodiment, the mobile working deck is configured to be lifted by the firing line hoist system, such that the BOP can be brought underneath the mobile working deck. The tower is preferably provided with a deck holding device at an elevated position along the tower to hold the deck in the raised, non-active position. It is noted that the working deck is configured for supporting substantially heavy loads, and therefore is quite heavy itself. The deck holding device and the tower should therefore be dimensioned to support the heavy working deck at a substantial height above the main deck. It is furthermore noted that the mobile deck is arranged pivotally, so that e.g. it can be brought in a vertical non-active position. For instance the mobile deck may have two pivotable deck parts.
Currently there is the need to transport multiple subsea devices, such as BOPs and subsea trees. The known drilling vessels provide at most two BOP parking positions, in which case the working deck is to be lifted by the firing line hoisting system to allow a BOP to be moved in the firing line, the lifting of the working deck limits the use of the firing line hoisting system.